Proliferation and differentiation of cells within multicellular organisms is controlled by hormones and polypeptide growth factors. These diffusable molecules allow cells to communicate with each other and act in concert to form organs, and to repair and regenerate damaged tissue. Examples of hormones and growth factors include the steroid hormones (e.g., estrogen, testosterone), parathyroid hormone, follicle stimulating hormone, the interleukins, platelet derived growth factor (PDGF), epidermal growth factor (EGF), granulocyte-macrophage colony stimulating factor (GM-CSF), erythropoietin (EPO), insulin and calcitonin.
Hormones and growth factors influence cellular metabolism by binding to receptors. Receptors may be integral membrane proteins that are linked to signaling pathways within the cell, such as second messenger systems. Other classes of receptors are soluble molecules, such as certain transcription factors.
Insulin belongs to a group of protein/polypeptide hormones. Insulin increases the rate of synthesis of glycogen, fatty acids, and proteins and stimulates glycolysis. It also promotes the transport of glucose, some other sugars, and amino acids into muscle and fat cells. The mature form of insulin consists of a 30 amino acid residue B chain, that is at the N-terminus of the propeptide form, and a 21 amino acid residue A chain, that is C-terminal. Proinsulin also contains a connecting peptide, C-peptide, between the B chain and A chain that is cleaved out to form mature insulin. The B chain and A chain are covalently joined by two disulfide bonds. The B-chain, C-peptide, A-chain motif is found in several other proteins including, relaxin (U.S. Pat. No. 4,835,251, incorporated herein by reference), insulin-like growth factors (IGF) I and II (Bang and Hall, In “Insulin-like Growth Factors”, P. N. Schofield (eds.), 151–177, Oxford Univ. Press, Oxford; incorporated herein by reference), Leydig factor (Bullesbach et al., J. Biol. Chem. 270:16011–16015, 1995; incorporated herein by reference), and early placenta insulin-like factor (EPIL; Chassin et al., Genomics 29:465–470, 1995, incorporated herein by reference). Unlike the other members of the insulin superfamily, IGF I and IGF II have D and E domains that are cleaved post-translationally. Cysteines that are involved in disulfide bonds are conserved in all the members of the family and play a role in the tertiary structure of the molecules.
Spermatogenesis is the process by which a germ cell proceeds through multiple stages of differentiation, and culminates in the formation of a terminally differentiated cell with a unique function. Hematopoiesis can be used as a paradigm for understanding spermatogenesis, and while there are striking parallels between what is known about hematopoiesis and spermatogenesis, the maturation of spermatogonia (germ cells) is less clearly understood than the maturation of hematopoietic stem cells. Particularly deficient is an understanding of factors that regulate the maturation process in spermatogenesis. Recent evidence suggests that some cytokines involved in the progression of stem cells of the hematopoietic lineage to fully differentiated cells are also involved in sperm cell maturation. In a fashion similar to cytokine action in hematopoiesis, these cytokines are thought to act at specific stages in the germ cell's maturation. For example, stem cell factor (also known as Steel factor and c-kit ligand) mRNA is expressed in spermatogonia (Manova et al., Development 110:1057–1066, 1990), and administration of a monoclonal antibody to stem cell factor to adult or prepubertal mice causes depletion of differentiating spermatogonia but has no effect on the non-differentiating spermatogonia, or spermatocytes (Yoshinaga et al., Development 113:689–699, 1991). Other cytokines that have been associated with spermatogenesis include IL-1, IL-6 and β-TGF (Sharp, Regulation of Spermatogenesis, in Knobil and Neil (ed.), Physiol. Reproduction, (2nd ed.), Raven, N.Y., 1994).
Because growth factors have had an enormous impact on our understanding of and ability to treat metabolic and cellular disorders, discovery of new factors is important. A new growth factor found in testis is particularly important for elucidating the spermatogenic process and potentially affecting the outcome of that process.